Download Topic 4

Bell ringer
An empty shopping cart is pushed along a grocery
store aisle at constant velocity.
1. Identify the forces on the same cart at rest.
2. While the cart is moving along an aisle, it comes in contact
with a smear of margarine that had recently been dropped on
the floor. Suddenly the friction force is reduced from -40.0
newtons to -20.0 newtons. What is the net force on the cart if the
“pushing force” remains at 40.0 newtons? Does the grocery cart
move at constant velocity over the spilled margarine?
3. Identify the normal force on the shopping cart after 75
newtons of groceries are added to the cart.
Solution
1. Forces: Weight and Normal force
2.The net force on the cart is +20 N. While the
cart is on the slippery margarine, it is not
moving at constant velocity since it is
experiencing a net force (acceleration).
3. The normal force on the cart was 105 N, so
after adding 75 N with groceries, the new
weight is +180 N, consequently the normal
formce is +180 N.
Chapter Six: Laws of Motion
6.1 Newton’s First Law
6.2 Newton’s Second Law
6.3 Newton’s Third Law and
Momentum
6.2 Newton’s second law
 Newton’s first law tells us that
motion cannot change without a
net force.
 According to Newton’s second law,
the amount of acceleration
depends on both the force and the
mass.
6.2 The newton
 The S.I. unit of
force (newton) is
defined by the
second law.
 A newton is the
amount of force
needed to
accelerate a 1 kg
object by 1m/s.
6.2 Newton’s second law
 There are three main ideas related
to Newton’s Second Law:
1. Acceleration is the result of
unbalanced forces.
2. A larger force makes a
proportionally larger acceleration.
3. Acceleration is inversely
proportional to mass.
6.2 Newton’s second law
 Unbalanced forces cause changes in
speed, direction, or both.
6.2 Acceleration and force
The second law says
that acceleration is
proportional to force.
If force is increased
or decreased,
acceleration will be
increased or
decreased by the
same factor.
6.2 Acceleration and direction
Another important factor of the second law
is that the acceleration is always in the
same direction as the net force.
6.2 Acceleration and mass
The greater the mass, the smaller the
acceleration for a given force.
This means acceleration is inversely
proportional to mass.
6.2 Acceleration, force and mass
The acceleration caused by a force is
proportional to force and inversely
proportional to mass.
The stronger the
force on an object,
the greater its
acceleration.
 Force is directly
proportional to
acceleration.
 If twice the force
is applied, the
acceleration is
twice as great.
The greater the
mass, the smaller
the acceleration
for a given force.
 Mass is
inversely
related to force.
 An object with
twice the mass
will have half
the acceleration
if the same
force is applied.

6.2 Applying the second law
Keep the following
important ideas in mind:
1. The net force is what
causes acceleration.
2. If there is no acceleration,
the net force must be
zero.
3. If there is acceleration,
there must also be a net
force.
4. The force unit of newtons
is based on kilograms,
meters, and seconds.
Solving Problems
 A car has a mass of 1,000 kilograms.
If a net force of 2,000 N is exerted
on the car, what is its acceleration?
1. Looking for:
 …car’s acceleration
2. Given
 …mass = 1,000 kg; net force = 2,000 N
3. Relationships:
 a=F/m
4. Solution:
 2, 000 N ÷ 1,000 kg = 2 N/kg = 2 m/s2